High vacuum



Zero Speed Sw'l'ch H. A. STEINHERZ ETAL 2,958,455

HIGH VACUUM Filed Nov. 2, 1956 0 E N Q q- O N E N fi Lm Ef; U 0 D u n. U3 ED (D o 'l' `\f N l )l "6 i, f N l. d

'U E N D U t J $2 Ld INVENTORS United States Patent O rifice HIGH VACUUM Hans A. Steinherz, Wellesley Hills, and George T. Blackbnm, Needham, Mass., assignors to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Filed Nov. 2, 1956, Ser. No. 620,134

43 Claims. (Cl. 230-45) This invention relates to a high vacuum device and in particular to multistage vacuum pumping apparatus. In recent years, due to the increased size of vacuum processing systems, the use of positive displacement blowers (such as Roots blowers) in pumping systems has become very desirable. These blowers have high volumetric efliciency and, therefore, can handle enormous quantities of gases at relatively low pressures. Such blowers are generally backed up by a conventional mechanical vacuum pump of considerably smaller volumetric pumping capacity.

In the past, numerous complex arrangements have been devised for by-passing the blower during the initial pump down of the system. This has been necessary due to the fact that the theoretical pumping capacity of the blower is so much greater than that of the backing pump (or second-stage pump, as it is sometimes called), that tremendous quantities of power are required in order to drive the blower at higher pressures. Therefore, as the pressure differential between the inlet and the outlet of the blower increases, the quantity of gas passing through the blower becomes insufficient to dissipate the large quantity of heat produced by the blower, often resulting in disastrous overheating. U.S. Patent 2,721,694 is illustrative of one system which is provided with a by-pass around the blower through which the system is evacuated during the initial pump down.

All such prior art systems, however, have been subject to rather severe disadvantages due to the inherent complexity and costliness of the blower by-pass arrrangement. In the present invention the disadvantages and problems inherent in the prior art have been either successfully solved, or circumvented.

Accordingly, it is a principal object of the instant invention to provide an improved multistage, high capacity, vacuum pumping system which is simple to operate and maintain.

Another object of the invention is to provide a high capacity vacuum pumping system which is relatively inexpensive. In particular, the present invention provides for the elimination of considerable piping, several valves and the attendant possibility of leaks therein.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplied in the following detailed description and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature `and objects of the invention, reference should be had to the following detailed description in conjunction with the accompanying drawing, which is a diagrammatic, schematic illustration of one preferred embodiment of the invention.

In the present invention, the vacuum pumping system includes a blower of the Roots type or of the axial ow type as illustrated in U.S. Patents 2,287,716 and 2,345,780, and a mechanical backing pump, the inlet of Patented Nov. 1, 1960 which is connected directly to the outlet of the blower. The blower preferably has a pumping capacity on the order of ten times the pumping capacity of the backing pump. Each pump has its own electric motor, the blower motor being of a size suflicient to drive the blower at maximum volumetric capacity only when the blower is operating as a vacuum pump (i.e., in the pressure range below about 15 mm. Hg abs. as measured at the blower inlet). In the present invention, the blower motor is not used to drive the blower until this low pressure has been attained in the pumping system. However, since the blower constitutes an impedance in the pumping system between the inlet of the backing pump and the system to be evacuated, it is desirable that the blower operate at relatively slow speed while the system is being pumped down to 10 mm. Hg abs. or thereabouts. This is preferably accomplished by allowing the blower to windmill during the initial pump down or roughing period during which only the backing pump is in power-on operation. As the pressure is reduced, the rate of windmilling decreases due to the decreased pressure dilferential existing across the blower. It is at about 10 mm. Hg abs. that windmilling becomes ineilicient. Therefore, means, such as a zero speed switch, are provided to start the blower motor when the windmilling has fallen below a predetermined level, indicating that the pressure has been reduced to about l0 mm. Hg. Thereafter, the blower is driven by the blower motor at high speed. If, at a later point in the cycle, the blower inlet pressure exceeds about 15 mm. Hg, the blower motor can be disconnected by meansy of its overload protection.

Referring now to the drawing, there is illustrated one specific preferred embodiment of the invention wherein the blower is indicated at 10 as having an inlet 12 which is connected to a system 14 to be evacuated. The blower has an outlet 16 which is coupled directly to the inlet 18 of the backing pump 20. The backing pump 20 is arranged to discharge air from the system directly to the atmosphere through outlet 22. The backing pump is driven by motor 24, while the blower 10 is driven by motor 26 coupled by means of belts 32 to the blower drive shaft 28. A zero speed switch 30, connected to blower motor 26, can be set to start the blower motor when the revolutions per minute of the blower drive shaft 28 fall below a preset value.

In one specific embodiment of the invention, the blower 10 has a capacity of 1000 c.f.m. and is arranged to be driven 'at a speed of 3000 r.p.m. with an inlet pressure range of 10-4 mm. Hg abs. to about 15 mm. Hg abs. and a compression ratio of about 10 to 1. The backing pump has a maximum capacity of c.f.m. at a speed of 335 rpm., with an inlet pressure range of about 10-4 to 760 mm. Hg abs., the outlet being to atmosphere. With this arrangement, the blower motor 26 is a 10 H.P. 1750 r.p.m. motor coupled by means of belts 32 to the blower drive shaft 28. Motor 24 is a 5 H P. 1750 rpm. motor directly coupled to the backing pump. The backing pump motor 24 is started first. This causes the backing pump 20 to operate at 335 rpm. and blower 10 to windmill at about 300 r.p.m. As the pressure in inlet 12 is reduced (due to the evacuation `of system 14) to a pressure on the order of about l() mm. Hg abs., and the windmilling decreases, the zero Speed switch 30 is energized and the blower motor 26 is started, thereby rapidly bringing drive shaft 28 and blower 10 up to the operating speed of 3000 r.p.m. If the speed of the shaft at any time decreases to less than about 30 rpm., the blower motor 26 is disconnected by means of its overload protection.

Obviously, minor changes may be made in the above described apparatus without departing from the scope of the invention. For example, although only a zero speed switch was speciically mentioned as a means for automatically starting the blower when the windmilling has fallen below a predetermined level, any device which has an electrical output in proportion to the speed of a shaft device is suitable for the purposes of this invention.

Also, in the preferred embodiment of'the invention described above, the shaft 28 and the belts-32 are permanently connected to the blower itself and, therefore, turn when the blower windmills. Under certain circumstances, this is undesirable and an overriding clutch can `be provided in order to disconnect the shaft, belts, etc. from the blower during the windmilling operation.

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompartying drawing, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. In a vacuum pumping system of the type which includes a blower and a mechanical backing pump, the inlet to the blower being connected to a system to be evacuated and the outlet of the backing pump being arranged to discharge to atmosphere and wherein separate motors are provided for driving the blower and backing pump, the improvement which comprises means for connecting the blower outlet directly to the backing pump inlet thereby causing said blower to windmill during the initial pump-down operation as substantially all of the air being pumped by the backing pump is drawn through the blower, said windmilling minimizing the impedance effect of the blower upon the flow of air while only said backing pump is being driven by its motor, and switching means responsive to the revolution of said blower so that when the rate of wind milling of said blower decreases below a predetermined value the switching means operates to supply power to the blower motor and thereby start said blower, thus permitting the blower to cooperate with the backing pump to increase the degree of vacuum over the predetermined degree of vacuum at which the blower starts to operate without the need for bypass piping.

2, In a vacuum pumping system of the type which includes a blower and a mechanical backing pump, the inlet to the blower being connected to a system to be evacuated and the outlet of the backing pump being ar- 4 ranged to discharge to atmosphere and wherein separate motors are provided for driving the blower and backing pump, the improvement which comprises means for connecting the outlet of the blower directly to the inlet of the backing pump so that all of the air being pumped by the backing pump is drawn through the blower, thereby causing said blower to windmill during the initial pumpdown operation while only said backing pump is in operation, and switching means comprising a zero speed switch actuated by a low revolution rate of said blower so that when the rate of rotation of said blower decreases below a predetermined value, the switching means operates to supply power to the blower motor and thereby operates said blower thus permitting the blower to aid the backing pump to achieve a high degree of vacuum without the need of bypassing the blower during pumpdown, but preventing the electrical energization of the blower at conditions Where its operation requires excessive power.

3. In a vacuum pumping system of the type which includes a blower and a mechanical backing pump, the inlet to the blower being connected to a system to be evacuated ates to supply power to the blower motor and therebyV start said blower, thus providing an arrangement whereby the pumping combination achieves high eciency in pumping through the range of atmospheric pressure to high vacuum without the need of bypass piping.

References Cited in the tile of this patent UNITED STATES PATENTS 1,706,443 Dekle Mar. 26, 1929 2,492,075 Van Atta Dec. 20, 1949 FOREIGNA PATENTS 680,001

Great Britain Oct. l, 1952` 

